Conventionally, substrates for flexible print wiring boards are produced by combining an electrically conducting material with an insulator such as a polyimide film or a polyester film with the intervention of an adhesive such as of an epoxy resin or an acrylic resin. However, a substrate for a substrate for a flexible print wiring board produced by such a method is inferior in heat resistance and flame resistance because of the intervention of an adhesive layer. In addition, the substrate suffers from a greater dimensional variation when the electrically conducting material is etched or when the substrate is subjected to a certain heat treatment, thereby causing trouble in subsequent steps.
To solve such drawbacks, an attempt is made to produce a flexible print substrate by forming a polyimide-based resin layer directly on an electrically conducting material without the intervention of an adhesive layer. For example, Japanese Unexamined Patent Publication No. 60-157286 (1985) proposes a method for producing a substrate for a flexible print wiring board, wherein a solution of a polyimide precursor having a specific structure is applied directly on an electrically conducting material and then cured. However, when the electrically conducting material is partly etched away for formation of circuitry on the substrate for a flexible print wiring board produced by this method, the substrate is significantly curled with its conductor-etched surface facing inward. This presents a problem that electronic components cannot accurately be mounted on the substrate in a subsequent step, for example, in an electronic component mounting step. To solve this problem, Japanese Unexamined Patent Publications No. 1-245586 (1989), No. 4-274385 (1992) and No. 8-250860 (1996) propose methods, in which a laminate consisting of a plurality of polyimide-based resin layers having different thermal expansion coefficients is formed as the insulative polyimide-based resin layer on the electrically conducting material. However, these methods still fail to satisfactorily solve the problem associated with the curling.